The prevalence of asymptomatic RSV infection is low during infancy (<10%), but high in school age children (50%) and adults (>75%). This is the conclusion from a family-based field study by the RSV Research Group in Kilifi. Researchers determined the presence, load and duration of RSV infection within 40 households in relation to in-house spreading of RSV. The high incidence of asymptomatic RSV infection in older children and adults suggests an important role of asymptomatic RSV infection in within family spread of disease. However, as viral loads were lower and duration of shedding was shorter, asymptomatic infection explained less of within family spread of RSV infection than symptomatic infections. This is the first study quantifying and carefully explaining the role of asymptomatic RSV infection in viral transmission opening the possibility of prevention by hygiene measures.